Purification of magnesium perchlorate

ABSTRACT

1. A METHOD OF TREATING A MAGNESIUM PERCHLORATE SOLUTION COMPRISING: (A) INTRODUCING A MAGNESIUM PERCHLORATE SOLUTION, CONTAINING IN EXCESS OF 50 PARTS PER MILLION AND LESS THAN ABOUT 3000 PARTS PER MILLION OF AT LEAST ONE IMPURITY SELECTED FROM THE GROUP CONSISTING OF CHLORIDE AND CHLORATE IONS, INTO AN ELECTROLYTIC CELL BETWEEN A CATHODE AND A ANODE; (B) PASSING AN ELECTRIC CURRENT THROUGH THE ELECTROLYTIC CELL FROM THE ANODE TO THE CATHODE AT A CELL VOLTAGE OF FROM ABOUT 4.0 TO 8.0 VOLTS AND AT AN ANODIC CURRENT DENSITY OF FROM ABOUT 50 TO 150 AMPS PER SQUARE FOOT; AND (C) MAINTAINING THE PH OF THE SOLUTION WITHIN THE RANGE OF FROM ABOUT 4.5 TO 6.5 WHEREBY SUBSTANTIALLY ALL THE IMPURITIES ARE CONVERTED TO PERCHLORATE IONS.

United States Patent PURIFICATION OF MAGNESIUM PERCHLORATE James Edward Reynolds, Oklahoma City, Okla., assignor to Kerr-McGee Chemical Corp., Oklahoma City, Okla. No Drawing. Filed Mar. 5, 1973 Ser. No. 338,409 Int. Cl. C01b 11/18, 11/26 U.S. Cl. 20482 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of purifying magnesium perchlorate. More particularly, this invention relates to a method of reducing the impurity content of a magnesium perchlorate solution contaminated with at least one impurity selected from the group consisting of chloride and chlorate ions. Broadly, the method comprises electrolyzing a magnesium perchlorate solution contaminated with chloride ions, chlorate ions or mixtures thereof whereby substantially all of the ions are converted to perchlorate ions.

BACKGROUND OF THE INVENTION Chlorates generally and sodium perchlorate in particular are staple articles of commerce. The principal method of producing sodium perchlorate is by the electrolytic conversion of a chlorate in an aqueous solution to a perchlorate by anodic oxidation. An example of such a process is disclosed in U.S. 2,392,769.

Sodium perchlorate produced by electrolysis frequently is used as an intermediate for the manufacture of other perchlorate products such as the potassium, ammonium, magnesium and lithium salts. These salts may all be made conveniently from sodium perchlorate by processes of metathesis or double decomposition; for example, see U.S. 2,512,973 for a method of producing potassium perchlorate.

More recently in U.S. 3,020,124 it is suggested that an aqueous solution of lithium chloride may be electrolytically oxidized to perchlorate ions.

Anhydrous magnesium perchlorate has been purchased commercially for use as a drying agent. In the commercial production of magnesium perchlorate, a magnesium salt such as the carbonate or oxide is reacted by double decomposition with a perchlorate salt, e.g., ammonium perchlorate. In these procedures, impurities are unavoidably introduced by both reactants and subsequent purification problems are diflicult.

Recently, a new use for magnesium perchlorate has been found as the electrolyte in a primary magnesium dry cell battery. To be acceptable for use in a dry cell battery, it is essential that the magnesium perchlorate be substantially free of impurities. More particularly, the magnesium perchlorate must not contain more than a total of 50 parts and preferably less than parts per million total of chlorate and chloride ions as impurities. Higher levels of these particular impurities seriously impair battery performance and storage capacity. It is obvious, therefore, that a method of reducing the chloride and chlorate content of magnesium perchlorate is desirable.

A method is described in French Patent 2,069,200 wherein pure reactants consisting of perchloric acid and magnesium metal are used to produce a pure magnesium perchlorate. Such a method is not economical, however, except in those applications Where extremely low impurity levels are a requirement. There is need, therefore, for a method of obtaining magnesium perchlorate substantially free of chloride and chlorate impurities.

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SUMMARY OF THE INVENTION The present invention now provides a method to reduce the chloride and chlorate content of a magnesium perchlorate solution. Broadly, the present invention comprises electrolytically oxidizing -an aqueous solution of magnesium perchlorate containing at least one impurity selected from the group consisting of chloride and chlorate ions whereby the total impurity level is reduced to less than 50 parts per million. More particularly, the present invention comprises introducing an impure magnesium perchlorate solution containing as high as 3000 parts per million of at least one impurity selected from the group consisting of chloride and chlorate ions into an electrolytic cell between a cathode and a platinum anode. A rectified electric current is passed through the cell from the anode to the cathode, at a cell voltage within the range of from about 4 to 8 volts and at an anodic current density within the range of from about 50 to amps per square foot while maintaining the pH of the perchlorate solution within a range of from about 4.5 to 6.5 whereby substantially all of the impurities are converted to perchlorate ions.

DESCRIPTION OF A PREFERRED EMBODIMENT In accordance with the present invention an aqueous solution of magnesium perchlorate, contaminated with at least one impurity selected from the group consisting of chloride and chlorate ions, is subjected to electrolysis whereby the impurities are converted to perchlorates. The present invention is applicable to magnesium perchlorate solutions containing chloride ions, chlorate ions or a mixture threof in concentrations as high as 3000 parts per million. It has been discovered that it is possible to reduce the impurity level of such a magnesium perchlorate solution to less than 20 parts per million.

It is surprising to discover that a solution containing such impurities at such low levels can be effectively electrolytically oxidized to perchlorates at a practical rate.

Conventional sodium perchlorate electrolytic cells operate with an electrolyte having sodium chlorate concentration above 3000 ppm. (3 g./l.) since at lower chlorate concentrations anode efliciency is seriously impaired, platinum consumption is uneconomically high and evolution of ozone is copious. Also, at low chlorate levels, an equilibrium chloride'level is sustained in the electrolyte presumably due to simultaneous reduction reactions at the cathode. Further, those skilled in perchlorate electrolysis would expect that when an alkaline earth such as magnesium perchlorate is electrolyzed it would result in the formation of insoluble magnesium hydroxide deposits on the cathode surfaces. Surprisingly, it has been discovered that such deposits are unexpectedly minimal in the magnesium perchlorate system.

As those versed in the art will appreciate, the particular apparatus utilized is not critical. More particularly, any electrolytic cell customarily used in the electrolysis of perchlorates from chlorates is applicable to the process of the present invention. An example of such a cell is disclosed in U.S. 2,512,973. It is preferred that the cell selected have a platinum anode and a steel cathode although any anode material such as, for example, lead dioxide or magnetite may be employed, providing the anode potential is high enough to favor perchlorate formation rather than oxygen evolution. The cathode, of course, may be made from any cathodically active material such as, for example, copper, nickel, graphite, platinum and the like. A steel cathode generally is preferred since steel is readily available and inexpensive.

A magnesium perchlorate solution containing at least one impurity selected from the group consisting of chloride and chlorate ions is introduced into the electrolytic cell between the cathode and anode. An electric current is passed through the electrolytic cell from the anode to the cathode at a voltage within the range of from about 4 to 8 volts and an anodic current density Within the range of from about 50 to 150 amps per square foot. Particularly good results have been obtained at a cell voltage of about 5 to 7 volts and an anodic current density of about 120 amps per square foot.

As those versed in the art will appreciate, such high current densities with the low concentrations of impurities contemplated herein necessarily result in a lower current efficiency for discharge of chloride and chlorate ions. An optimum current efiiciency for a concentration of, for example, chloride ions within the range of from 500 ppm. to 20 ppm. is obtained with a current density of about amps per square foot. However, for economic reasons (such as, for example, to reduce the time required for electrolysis) it generally is desirable to operate at higher current densities, namely, from about 50 to 150 amps per square foot.

Too low an anode current density (below about 50 amps per square foot) permits a competing side reaction, i.e., a reduction of chlorate and hypochlorite ions, to proceed at such a rate as to offset the rate of depletion of chlorate and chloride by anodic discharge. The optimum anode current density for minimum power consumption is, of course, dependent upon such factors as cell geometry as well as the usual electrolysis variables. In the interests of high cell productivity, it is found most desirable to operate at high anode current densities since power consumption is not a significant economic factor.

It is essential that the pH of the solution, during electrolysis, be maintained within the range of from about 4.5 to 6.5 and preferably within the range of from about 5.0 to 6.0. Obviously, of course, the discharge of oxygen at the anode during electrolysis will cause a gradual increase in the acidity of the solution. To maintain the pH within the desired range, small amounts of a basic material are added to the electrolyte. A basic magnesium salt can be added to control acidity to the proper level. The carbonate or hydroxide of magnesium is an inexpensive commercially available salt and is, therefore, preferred.

The length of time required to reduce the impurity content to less than p.p.m. will depend upon the initial impurity concentration, batch size, amperage, anodic current density during electrolysis and the like. Generally, the time required will be within the range of from about 10 to 60 hours.

The temperature of the electrolyte during electrolysis is not critical. The electrolytic cell may be operated at ambient or any other convenient temperature but preferably is operated at a temperature within the range of from about 20 to 60 C. Particularly good results have been obtained when the temperature has been maintained within the range of from about to C.

The following example is set forth to further illustrate the process of thepresent invention.

Example I A batch operation is carried out in an electrolytic cell having platinum clad titanium anodes and steel cathodes. The initial electrolyte comprises approximately 500 gallons of a 48% by weight magnesium perchlorate solution contaminated with about 600 parts per million total of a mixture of chlorate and chloride ions. The electrolysis is conducted at a cell voltage of about 6.5 volts and at an anodic current density of about 120 amps per square foot. The pH of the solution is maintained at about 5-5.5

by the addition to the electrolyte of small quantities of magnesium hydroxide. The temperature during electrolysis is within the range of from about 35 to 45 C.

After about 60 hours of electrolysis a sample of the electrolyte is obtained and analyzed. The results of the analysis show that the total chloride and chlorate content now is less than 20 parts per million, thus, demonstrating the eflicacy of the present invention to reduce the chloride and chlorate content of a magnesium perchlorate solution.

' What is claimed is:

1. A method of treating a magnesium perchlorate solution comprising:

(a) introducing a magnesium perchlorate solution, containing in excess of 50 parts per million and less than about 3000 parts per million of at least one impurity selected from the group consisting of chloride and chlorate ions, into an electrolytic cell between a cathode and a anode;

(b) passing an electric current through the electrolytic cell from the anode to the cathode at a cell voltage of from about 4.0 to 8.0 volts and at an anodic current density of from about 50 to 150 amps per square foot; and

(c) maintaining the pH of the solution within the range of from about 4.5 to 65 whereby substantially all the impurities are converted to perchlorate ions.

2. The method of claim 1 wherein the solution is maintained at a temperature within the range of from about 20 to 60 C.

3. The method of claim 1 wherein the anodic current density is about amps per square foot.

4. The method of claim 1 wherein the cell voltage is about 5 to 7 volts.

5. The method of claim 1 wherein the pH of the solution is maintained within the range of from about5to 6.

6. The method of claim 1 wherein the electric current is passed through the electrolytic cell from the cathode to the anode for a time within the range of from about 10 to 60 hours.

7. The method of claim 6 wherein the cell voltage is maintained at about 5 to 7 volts.

8. The method of claim 7 wherein the pH of the solution is maintained at from about 5 to 6.

. 9. The method of claim 8 wherein the anodic current density is maintained at about 120 amps per square foot.

10. The method of claim 1 wherein the cell voltage is maintained at about 5 to 7 volts; the pH of the solution is maintained within the range of from about 5 to 6 and the anodic current density is maintained at about 120 strips per square foot.

References Cited UNITED STATES PATENTS 2,772,229 11/1956 Karr 204-82 3,020,124 2/1962 Bravo et ah 204-82 X 3,493,478 2/ 1970 Udupa et a1. 20482 OTHER REFERENCES Izgaryshev, Chemical Abstracts, Vol. 44, columns 57 26, 5727 (1950).

F. C. EDMUNDSON, Primary Examiner U.S. Cl. X.R. 423-476 

1. A METHOD OF TREATING A MAGNESIUM PERCHLORATE SOLUTION COMPRISING: (A) INTRODUCING A MAGNESIUM PERCHLORATE SOLUTION, CONTAINING IN EXCESS OF 50 PARTS PER MILLION AND LESS THAN ABOUT 3000 PARTS PER MILLION OF AT LEAST ONE IMPURITY SELECTED FROM THE GROUP CONSISTING OF CHLORIDE AND CHLORATE IONS, INTO AN ELECTROLYTIC CELL BETWEEN A CATHODE AND A ANODE; (B) PASSING AN ELECTRIC CURRENT THROUGH THE ELECTROLYTIC CELL FROM THE ANODE TO THE CATHODE AT A CELL VOLTAGE OF FROM ABOUT 4.0 TO 8.0 VOLTS AND AT AN ANODIC CURRENT DENSITY OF FROM ABOUT 50 TO 150 AMPS PER SQUARE FOOT; AND (C) MAINTAINING THE PH OF THE SOLUTION WITHIN THE RANGE OF FROM ABOUT 4.5 TO 6.5 WHEREBY SUBSTANTIALLY ALL THE IMPURITIES ARE CONVERTED TO PERCHLORATE IONS. 